According to the typical thermal structure of the ocean, the water column can be divided into three layers: the mixed layer, the thermocline and the deep layer. In this study, we provide a new methodology, based on a function adjustment to the temperature profile, to locate the minimum and maximum depths of the strongest thermocline. We first validated our methodology by comparing the mixed layer depth obtained with the method proposed here with three other methods from previous studies. Since we found a very good agreement between the four methods we used the function adjustment to compute the monthly climatologies of the maximum thermocline depth and the thermocline thickness and strength in the global ocean. We also provide an assessment of the regions of the ocean where our adjustment is valid, i.e., where the thermal structure of the ocean follows the three-layer structure. However, there are ocean regions where the water column cannot be separated into three layers due to the dynamic processes that alter it. This assessment highlights the limitations of the existing methods to accurately determine the mixed layer depth and the thermocline depth in oceanic regions that are particularly turbulent such as the Southern Ocean and the northern North Atlantic, among others. The method proposed here has shown to be robust and easy to apply.

Currently there is a huge amount of freely available hydrographic data, and it is increasingly important to have easy access to it and to be provided with as much information as possible. Argo is a global collection of around 4000 active autonomous hydrographic profilers. Argo data go through two quality processes, real time and delayed mode. This work shows a methodology to filter profiles within a given polygon using the odd–even algorithm; this allows analysis of a study area, regardless of size, shape or location. The aim is to offer two filtering methods and to discard only the real-time quality control data that present salinity drifts. This takes advantage of the largest possible amount of valid data within a given polygon. In the study area selected as an example, it was possible to recover around 80 % in the case of the first filter that uses cluster analysis and 30 % in the case of the second, which discards profilers with salinity drifts, of the total real-time quality control data that are usually discarded by the users due to problems such as salinity drifts. This allows users to use any of the filters or a combination of both to have a greater amount of data within the study area of their interest in a matter of minutes, rather than waiting for the delayed-mode quality control that takes up to 12 months to be completed. This methodology has been tested for its replicability in five selected areas around the world and has obtained good results.

The influence of El Niño–Southern Oscillation (ENSO) on productivity at upwelling systems has been extensively studied. However, in recent decades, ENSO diversity has been documented; there are more frequent events with anomalies in the central Pacific (CP) causing ecological impacts that are different from the canonical events centered in the eastern Pacific (EP). We hypothesize that ENSO effects on upwelling intensity and its biological response are highly dependent on ENSO diversity. Wavelet coherence was computed for monthly standardized anomalies of Ekman transport and sea surface Chlorophyll-a (Chl-a) concentration from eight upwelling centers located along the west coast of the Baja California peninsula (1993–2022). Ekman transport (ET) showed a non-linear association with ENSO at interannual periodicities between 1.2 and 7 years, as well as semiannual scale variability. Coherence between ENSO and ET showed a reduction in upwelling intensity during CP ENSO events and an increased upwelling intensity after EP El Niño events with regional differences. Our results reveal an influence of northern Pacific circulation that subsequently influences ENSO events during its development and its peak. We detected an influence of SST and wind anomalies on the ENSO–Ekman transport connection associated with the northern Pacific Meridional Mode pattern. The CP shows an important role in sea surface Chl-a concentration anomalies (1998–2022). We highlight the conceptual relevance of distinguishing ENSO diversity, with influential ecological effects associated with CP ENSO events.

Larval fish habitats and their representative species were analyzed in relation to water mass distribution and mesoscale processes at the entrance of the Gulf of California during April 2012. Results show that the oxygenated Gulf of California Water (GCW) spread south near Cabo Corrientes, above the hypoxic (>44 μmol kg-¹) Subtropical Subsurface Water (StSsW), causing strong stratification. Two mesoscale eddies south of Cabo San Lucas entrapped surface oxygenated Transitional Water and sunk hypoxic waters to ~100 m depth. These conditions defined two larval fish habitats that extended horizontally over most of the study area and were vertically separated by the eddy-modulated oxycline depth: a surface habitat associated with the southwards expansion of GCW and the influence of the eddies, represented by Vinciguerria lucetia; and a hypoxic habitat, represented by the highest abundances of Diogenichthys laternatus. Results are in contrast with previous studies that recorded these mesopelagic species throughout the water column. A third larval fish habitat was associated with the upwelling area off Cabo Corrientes, represented by Bregmaceros bathymaster. This species shows high tolerance to wide gradients of dissolved oxygen. Our records indicate that a highly stratified water column could favor the expansion of hypoxic StSsW and associated marine biota.